The present invention relates generally to strain gauges and more specifically to a spark plug mounted thick film strain gauge that allows the pressure in an engine cylinder to be monitored.
A strain gauge mounted to a spark plug shell yields an excellent representation of the engine cylinder pressure with high signal-to-noise ratio. However, commercial strain gauges have a gauge factor of approximately 2. A gauge factor of 2 will provide a typical signal level of only a few microvolts. Using a commercially available strain gauge would require a very high degree of amplification. Strain gauges fabricated from thick-film resistors have gauge factors of between 10 to 20, which produces a signal level which is 5 to 10 times greater than commercially available strain gauges.
What remains in the art is a spark plug mounted thick film strain gauge which provides a higher magnitude of electrical signal than that of commercially available strain gauges.
The present invention is a spark plug mounted thick film strain gauge which requires less amplification than that of the prior art. The thick film strain gauge is applied to a metal (ie., stainless steel) shell of a spark plug. There are two preferable ways of applying a thick film strain gauge to the metal shell in axial and circumferential orientations. In a first embodiment, a thick film resistor may be directly printed onto the metal shell with special screen printing equipment. Each layer of the thick film resistor is printed, allowed to dry, and fired in a kiln to fix the ink to the metal shell, or the previous layer. In a second embodiment, a thick film resistor is printed and applied as a decal to the metal shell. The spark plug is then fired in a kiln to fix the thick film resistor to the metal shell.
The thick film resistors may be included in a quarter, a half, or a full wheatstone bridge strain gauge circuit. A single thick film resistor is used in a quarter wheatstone bridge circuit; two thick film resistors are used in a half wheatstone bridge circuit; and four thick film resistors are used in a full wheatstone bridge circuit. The full wheatstone bridge circuit offers a signal output which is multiplied by as much as four times, and the half wheatstone bridge circuit offers a signal output which is multiplied by as much as two times, the output of the quarter wheatstone bridge circuit. With a full wheatstone bridge circuit, four thick film resistors must be applied to the metal shell.
An automatic drift compensating circuit is used to monitor the change in resistance experienced by the thick film resistors affixed to the spark plug. The automatic drift compensating circuit includes an input amplifier, a compensating amplifier and an output amplifier. The output of the wheatstone bridge circuit is input to the input amplifier. The output of the input amplifier is input to the compensation amplifier. The output of the input amplifier and the compensation amplifier is input to the output amplifier. The output amplifier outputs a voltage signal which is proportional to the pressure changes occurring inside the engine cylinder into which the spark plug has been threaded. A second embodiment of an automatic drift compensating circuit includes a first input amplifier, a second input amplifier, and an output amplifier. The output of the wheatstone bridge circuit is input into the first and second input amplifiers. The outputs of the first and second input amplifier are input into the output amplifier.
Accordingly, it is an object of the present invention to provide a spark plug mounted thick film strain gauge which provides a higher magnitude electrical signal than commercially available strain gauges.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.